1. Field of the Invention
Exemplary aspects of the present invention generally relate to a fixing device and an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including any combination thereof, and more particularly, to an image forming apparatus including the fixing device.
2. Description of the Background Art
A related art fixing device for an image forming apparatus, for example, that according to JP-H08-27571-B, is equipped with a heater, a film member that slidably moves with the heater, and a pressure member that is movable and presses against the heater through the film member.
In this fixing device, a recording medium bearing an unfixed toner image is sandwiched between the film member and the pressure member, and transported.
The place where the film member and the pressure member meet and press against each other is a so-called nip portion. In the related art fixing device, for various reasons a width of the heater is typically less than a width of the nip portion.
A drawback to the foregoing configuration is that, because the heater heats the recording medium through the film member, an undesirable amount of force is applied to the nip portion when paper jams occur and/or the heater is damaged when a foreign substance accidentally enters the nip portion.
Furthermore, when the recording medium, the width of which is narrower than the width of the nip portion, passes through the nip portion, a temperature of end portions of the heater rises significantly because there is no media present thereat to absorb the heat thus generated, thereby damaging the heater. In a case in which a primary current flows through the heater disposed substantially behind the thin film member, if the heater is damaged, a problem associated with electrical safety arises.
Another example of a known fixing device, disclosed in JP-S58-190659-U, is a belt-type fixing device in which a heater is disposed immediately before a nip portion between a pair of rollers, around which an endless belt is wound.
In this fixing unit, the endless belt is heated, and fixing is performed in the nip portion between the pair of rollers.
With this configuration, although the endless belt can be heated relatively fast, the pair of rollers is not easily heated because a heat capacity of the rollers is relatively large. Consequently, it is barely possible to raise a fixing temperature from room temperature to an appropriate temperature for fixing quickly when fixing needs to be performed.
In order to achieve a desired temperature in a short period of time, electric power needs to be applied to the heater constantly so as to keep the rollers heated to the desired temperature even if no recording medium passes between the nip portion, thereby defeating the purpose of reducing power consumption and improving image fixability.
Another example of a known fixing device, disclosed in Japanese Patent No. 3835298, is equipped with a carbon-based heater in which a heating value or an amount of radiant heat is changed by providing holes at predetermined locations in a longitudinal direction of a heating element of the carbon-based heater, in this case, a carbon lamp, to change a resistance value of the heater.
In Japanese Patent No. 3835298, although a method of changing the heating value or the amount of radiant heat of the carbon lamp is disclosed, the actual use of the carbon lamp is not disclosed.
In general, in a heating device used in a known fixing unit, power is often supplied from an end portion of a heating element of the heating mechanism in the longitudinal direction thereof. For this reason, in order to secure a heating area, a length of the heater tends to be relatively long.
Furthermore, in a case in which the heating device is disposed inside the pressure member or the fixing member, heat escapes from the end portions of the heating element to the outside of the fixing member and the pressure member. The amount of heat slipping through the end portions of the heating element is greater than the amount of heat escaping from the center thereof. For this reason, the temperature at the end portions of the pressure member or the fixing member drops easily, causing fixing failure.